Polypeptides capable of transforming normal cells in vitro have been isolated from neoplastic and non-plastic cells. Several of these transforming growth factors (TGFs) are available in the quantity and purity necessary for structural investigations. We are defining the primary structure of these compounds using non-routine amino acid analysis and automated microsequencing studies of the intact molecule and fragments obtained after enzyme digestion. Amino acid analyses have been obtained on TGFs isolated from bovine kidney and human platelet placenta. Sequence analysis of TGF isolated from human placenta has produced assignments for 27 of 29 residues. These results are consistent with the amino acid analysis performed on this compound. The immediate goal is to determine the primary sequence of TGF to facilitate the development of a TGF-beta antagonist and to assist others in the study of TGF-beta molecular biology.